The implantation of stents has become established as one of the most effective therapeutic measures for the treatment of vascular diseases. Stents have the purpose of assuming a supporting function in hollow organs of a patient. For this purpose, stents featuring conventional designs have a filigree supporting structure comprising metal struts, which is initially present in compressed form for introduction in the body and is expanded at the site of the application. One of the main application areas of such stents is to permanently or temporarily widen and hold open vascular constrictions, particularly constrictions (stenosis) of coronary blood vessels. In addition, aneurysm stents are known, which are used primarily to seal the aneurysm.
Stents have a peripheral wall with sufficient load-bearing capacity to hold the constricted vessel open to the desired extent, and a tubular base body through which blood continues to flow without impairment. The peripheral wall is generally formed by a lattice-like supporting structure, which allows the stent to be introduced in a compressed state, in which it has a small outside diameter, all the way to the stenosis to be treated in the particular vessel and to be expanded there, for example by way of a balloon catheter, so far until the vessel has the desired, enlarged inside diameter. As an alternative, shape memory materials such as nitinol have the ability to self-expand when a restoring force keeping the implant at a small diameter is eliminated. The restoring force is generally applied to the material by a protective tube.
The stent has a base body made of an implant material. An implant material is a non-living material, which is employed for applications in medicine and interacts with biological systems. A basic prerequisite for the use of a material as implant material, which is in contact with the body area when used as intended, is the body friendliness thereof (biocompatibility). For the purpose of the present application, biocompatibility shall be understood to mean the ability of a material to induce an appropriate tissue reaction in a specific application. This includes an adaptation of the chemical, physical, biological, and morphological surface properties of an implant to the recipient's tissue with the aim of a clinically desired interaction. The biocompatibility of the implant material is also dependent on the temporal course of the reaction of the biosystem in which it is implanted. For example, irritations and inflammations occur in a relatively short time, which can lead to tissue changes. Depending on the properties of the implant material, biological systems thus react in different ways. According to the reaction of the biosystem, the implant materials can be divided into bioactive, bioinert and degradable/resorbable materials.
Implant materials comprise polymers, metallic materials, and ceramic materials (as coatings, for example). Biocompatible metals and metal alloys for permanent implants comprise, for example, stainless steels (such as 316L), cobalt-based alloys (such as CoCrMo cast alloys, CoCrMo forge alloys, CoCrWNi forge alloys and CoCrNiMo forge alloys), technical pure titanium and titanium alloys (such as cp titanium, TiAl6V4 or TiAl6Nb7) and gold alloys. In the field of biocorrodible stents, the use of magnesium or technical pure iron as well as biocorrodible base alloys of the elements magnesium, iron, zinc, molybdenum, and tungsten are proposed. The present invention relates to permanent implant materials, in particular cobalt-based alloys.
Stents desirably have the ability to tolerate extensive plastic strain and maintain the size or diameter thereof when they are expanded. In general, the ideal stent should:                have a low profile; this includes the suitability of being crimped onto a balloon catheter;        exhibit good expansion properties; when the stent is introduced in the lesion and the balloon is inflated, the stent should uniformly expand so as to adapt to the vessel wall;        have sufficient radial strength and negligible recoil; once the stent has been placed, it should withstand the restoring forces of the atherosclerotic vessel wall and not collapse;        have sufficient flexibility; the stent can thus also be delivered through vessels and stenoses having smaller diameters or narrow radii;        have adequate radiopacity or MRI compatibility; the medical staff can thus assess the implantation and position of the stent in vivo;        have low thrombogenicity; the material should be biocompatible and in particular prevent the deposition and agglutination of platelets; and        have the option of releasing active agents; this is used in particular to prevent restenosis.        
These requirements address in particular the mechanical properties of the material of which the stent is produced. The classic 316L, MP53N and L-605 materials used for constructing balloon-expandable stents have mechanical drawbacks which restrict the freedom in stent design development and in use:                (i) insufficient (tensile) strength and plastic expansion         As a result, the collapse pressure and radial strength are lower, necessitating thicker stent struts, which results in a thicker crimping profile and a larger loss of lumen during implantation, delays healing (endothelialization) into the vascular wall and restricts the freedom in the geometric stent design development. Thicker struts additionally make the stent more rigid, and in the crimped state the stent profile (diameter) increases, which reduces the flexibility around bends within the vascular system.        (ii) elastic limit Rp0.2 too high         This results in high elastic recovery, which worsens the crimpability, leads to a thicker crimp profile, lowers the stent retaining force on the balloon and causes higher recoil (loss of lumen due to expansion).        
Moreover, the biocompatibility of the material must be ensured. Nickel has been suggested as causing allergies or local and systemic incompatibilities in some instances. A need therefore exists for nickel-free materials, or at least for materials having a low nickel content, for medical use.
Although various nickel-free implant constructions have been proposed, they leave various problems and deficiencies unresolved.